-A-
Acid Rain - Acid deposition - commonly called acid rain - is caused by emissions of sulphur dioxide and nitrogen oxides. Although natural sources of sulphur oxides and nitrogen oxides do exist, more than 90% of the sulphur and 95% of the nitrogen emissions occurring in eastern North America are of human origin. These primary air pollutants arise from the use of coal in the production of electricity, from base-metal smelting, and from fuel combustion in vehicles. Once released into the atmosphere, they can be converted chemically into such secondary pollutants as nitric acid and sulfuric acid, both of which dissolve easily in water. The resulting acidic water droplets can be carried long distances by prevailing winds, returning to Earth as acid rain, snow, or fog.
Advection - Transport of an atmospheric property by the wind. See cold advection, moisture advection, warm advection.
Air Mass - An air mass, by definition, is a large dome of air which has similar horizontal temperature and moisture characteristics. Often, a front separates two different air masses. Fronts are very narrow zones of transition. In other words, temperatures can change dramatically with short horizontal distances near fronts. Fronts are usually anywhere from 10 kilometers to hundreds of kilometers wide, while air masses can be thousands of kilometers wide.
Air Pressure - The force exerted on a surface by the weight of the air above it. The internationally recognized unit for measuring this pressure is the kilopascal.
Anemometer - An instrument to measure wind speed. Wind directions are always reported as the direction winds are coming from - a southerly wind pushes air from the south to the north.
Anticyclonic Rotation - Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.
Atmosphere - the air surrounding and bound to the earth. The mass of air held close to the earth by gravity. The atmosphere is subdivided into four sections: the troposphere- from the earth's surface to an altitude of about 10 km; the stratosphere - from 10 km to 50 km; the mesosphere - from 50 km to 80 km; and the thermosphere- beyond 80 km.
Aurora Borealis - also known as the northern lights - The luminous, radiant emission from the upper atmosphere over middle and high latitudes, and centred around the earth's magnetic poles. These silent fireworks are often seen on clear winter nights in a variety of shapes and colors.
Automated Weather Station - An unmanned station with various sensors that measure weather elements such as temperature/wind/pressure and transmit these readings for use by meteorologists.
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Backing Winds - Winds which shift in a counterclockwise direction with time at a given location (e.g. from southerly to southeasterly), or change direction in a counterclockwise sense with height (e.g. westerly at the surface but becoming more southerly aloft). The opposite of veering winds.
Baroclinic Zone - A region in which a temperature gradient exists on a constant pressure surface. Baroclinic zones are favored areas for strengthening and weakening systems.
Barometer - An instrument used to measure air pressure. The international standard of measurement is the kilopascal although millibars and inches of mercury are also commonly used.
Blizzard - A severe storm lasting four or more hours. It is characterized by low temperatures, strong winds and poor visibility due to blowing snow. True blizzard conditions are most common on the prairies of Canada and the United States. Blizzards are a rare occurrence on the west coast and in Atlantic Canada. The Environment Canada has listed the following general criteria for issuing blizzard warnings in Canada: visibility less than 1 kilometer, wind speeds greater than 40 kilometers per hour, high windchill values
Boundary Layer - In general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
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There is a thin layer immediately above the earth's surface known as the surface boundary layer (or simply the surface layer). This layer is only a part of the planetary boundary layer, and represents the layer within which friction effects are more or less constant throughout (as opposed to decreasing with height, as they do above it). The surface boundary layer is roughly 10 meters thick, but again the exact depth is indeterminate. Like friction, the effects of insolation and radiational cooling are strongest within this layer.
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Chinook - Chinooks occur when a mountain range is exposed to a strong winds blowing at right angles, or near right angles to the direction of the mountain ridge. Moist air is forced up the mountains bringing both cloud and precipitation to the windward side. The descending air then becomes warmer and drier as it is forced down the leeward side of the mountains. The relatively warm, dry gusty winds that occasionally occur to the leeward side of mountain ranges around the world are known by many names. In Canada and the northern United States, they are referred to as Chinooks. In the southern states, they are known as Santa Ana and in parts of Europe, foehn winds.
Climate - The prevalent or characteristic meteorological conditions, and their extremes, of any place or region.
Cloud - A visible cluster of tiny water and/or ice particles in the atmosphere.
Cloud Formations - please see this online cloud atlas for photos and descriptions of all the various cloud types.
Cirrus - High-level clouds (5,000 meters or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.
Cold Advection - Transport of cold air into a region by horizontal winds.
Cold-air Funnel - A funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.
Cold Front - The leading edge of a cooler airmass.
Cold Pool - A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.
Comma Cloud - A synoptic scale cloud pattern with a characteristic comma-like shape, often seen on satellite photographs associated with large and intense low-pressure systems.
Condensation - The physical process by which vapour becomes liquid or solid; the opposite of evaporation.
Convection - Generally, transport of heat and moisture by the movement of a fluid. In meteorology, the term is used specifically to describe vertical transport of heat and moisture, especially by updrafts and downdrafts in an unstable atmosphere. The terms "convection" and "thunderstorms" often are used interchangeably, although thunderstorms are only one form of convection. Cbs, towering cumulus clouds are visible forms of convection. However, convection is not always made visible by clouds. Convection which occurs without cloud formation is called dry convection, while the visible convection processes referred to above are forms of moist convection.
Coriolis Effect - In synoptic scale weather systems (hurricanes and large mid-latitude storms), the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. The air flowing around a hurricane spins counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere (as does the earth, itself). In both hemispheres, this rotation is called cyclonic. If the earth did not rotate, the air would flow directly in towards the low pressure center, but on a spinning earth, the Coriolis force results in the are arcing in towards the low pressure center. The coriolis force is of much too small a magnitude to have any relevance to the direction of rotation in a sink or toilet.
Cumulus - Detached clouds, generally dense and with sharp outlines, showing vertical development in the form of domes, mounds, or towers. Tops normally are rounded while bases are more horizontal. See Cb, towering cumulus.
Cyclogenesis - Development or intensification of a low-pressure center (cyclone).
Cyclone - In the Northern Hemisphere, a closed counter-clockwise movement of air-known as a circulation - around a low pressure centre; usually called a low The term is frequently misused to describe a tornado.
*Cyclonic Circulation (or Cyclonic Rotation) - Circulation (or rotation) which is in the same sense as the Earth's rotation, i.e., counterclockwise (in the Northern Hemisphere) as would be seen from above. Nearly all storms and strong or violent tornadoes exhibit cyclonic rotation, but some smaller vortices, sometimes rotate anticyclonically (clockwise). Compare with anticyclonic rotation. -D-
Dew Point- The temperature air must be cooled in order for due or saturation to occur. Saturation will result in a cloud.
Diurnal - Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night).
Doppler Radar - Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).
Downdraft - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm.
Downstream - In the same direction as a stream or other flow, or toward the direction in which the flow is moving.
Drizzle - Precipitation from stratus clouds consisting of numerous minute, fine water droplets which appear to float. In drizzle, the droplets are much smaller than in rain.
Drought - an extended period of dry weather.
Dust Devil - A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather.
Dust Whirl - A rotating column of air rendered visible by dust.
Dynamics - Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.
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El Nino- An extensive ocean warming that begins along the coast of Peru and Ecuador and extends along the equator. A typical El Nino brings above average rainfall to the Southwest. The name El Niño, from the Spanish for "the little boy", refers to the Christ child, because the phenomenon is usually noticed around Christmas time in the Pacific Ocean off the west coast of South America.
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Freezing Rain - Rain which freezes on impact to form a coating of ice upon the ground and on the objects it strikes. A freezing rain warning is usually issued when slippery driving and walking conditions are expected, and/or when freezing rain may damage trees, power lines, or other structures.
Front - A boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.
Frost - Water vapour which deposits directly as a solid on a surface colder than the surrounding air and which has a temperature below freezing. It is not frozen dew. A Killing Frost is a frost severe enough to end the growing season.
Fujita Scale (or F Scale) - A scale of wind damage intensity in which wind speeds are inferred from an analysis of wind damage:
F0 (weak): 40- 72 mph, light damage.
F1 (weak): 73-112 mph, moderate damage.
F2 (strong): 113-157 mph, considerable damage.
F3 (strong): 158-206 mph, severe damage.
F4 (violent): 207-260 mph, devastating damage.
F5 (violent): 261-318 mph, (rare) incredible damage.All tornadoes, and most other severe local windstorms, are assigned a single number from this scale according to the most intense damage caused by the storm.
Funnel Cloud - Cloud extending from the base of a towering cumulus or Cb, associated with a rotating column of air that is not in contact with the ground (and hence different from a tornado). A condensation funnel is a tornado, not a funnel cloud, if either a) it is in contact with the ground or b) a debris cloud or dust whirl is visible beneath it.
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Greenhouse Effect - The greenhouse effect is the name applied to the process which causes the surface of the earth to be warmer than it would have been in the absence of an atmosphere because it receives energy from two sources: the sun and the atmosphere. Radiation is not trapped,and the atmosphere does not behave as a greenhouse and the greenhouse gasses do not behave as a blanket ~ the name "greenhouse effect" is somewhat of a misnomer..
Gust - A sudden, brief increase in wind speed, generally less than 20 seconds.
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Heat Lightning- Distant lightning that illuminates the sky but is too far away for it's thunder to be heard.
High Pressure- An area of high pressure or anticyclonic around which the wind blows in a clockwise motion in the Northern Hemisphere. The air is typically sinking at the middle of the anticyclone and therefore weather conditions are hot with clear skies.
Humidity - Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.
Hurricane - (also known as (Typhoon, Tropical Cyclones, Willy- Willies) Tropical storms with wind speeds of 64 knots (117km/h) up to 240 knots (414 km/h) that can be thousands of square kilometers in size. Such systems usually have a lifespan of several days. In the North Atlantic, the hurricane season is from May to November, but the majority of storms occur in August, September and October. Although the east coast is the area of Canada most frequented by hurricanes, these storms still average less than one per year over the Atlantic Provinces and coastal waters. While there have been as many as five in one year, several years can pass with no tropical storms. A hurricane warnng is issued in the Marine Forecast if winds are expected to exceed 64 knots (115 km/h).
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Jet Stream - Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.
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La Nina is the name for the cold phase of ENSO (El Nino Southern Oscillation), during which the cold pool in the eastern Pacific intensifies and the trade winds strengthen. The name La Niña originates from Spanish, meaning "the little girl", analogous to El Niño meaning "the little boy". It has also in the past been called anti-El Niño.
Low Pressure- An area of low pressure or cyclone, which the winds blow in a counter clockwise motion in the Northern Hemisphere. Weather associated with Low Pressure is clouds, windy conditions, cooler weather and many times rain.
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Microburst- A strong localized downdraft less than 4 km wide that occurs beneath thunderstorms. A strong downburst greater than 4 km is called a macro burst.
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Nocturnal - Related to nighttime, or occurring at night.
Normal - The long-term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.
Nowcast - A short-term weather forecast, generally out to six hours or less.
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Occluded Front- A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air behind the front is colder that the air ahead of it, the front is call a cold occlusion.
Orographic - Related to, or caused by, physical geography (such as mountains or sloping terrain).
Orographic Lift - Lifting of air caused by its passage up and over mountains or other sloping terrain.
Outflow Winds - Winds that blow down fjords and inlets from the land to the sea. When cold arctic air flows from the interior of BC onto the coast, the windspeeds through mainland inlets can reach over 100 km/h.
Ozone -A pungent-smelling, slightly bluish gas which is a close chemical cousin to molecular oxygen. About 90% of the earth's ozone is located in a natural layer far above the surface of the globe, in a frigid region of the atmosphere known as the stratosphere. Here in this outer region it protects the earth and its inhabitants from the harmful effects of ultraviolet radiation from the sun.
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Relative Humidity- The ratio of the amount of water vapor in the air compared to the amount required for saturation.
Ridge- An elongated area of high atmospheric pressure associated with the jet stream. High pressure and warm weather are typical conditions of a ridge.
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Saturation- An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure.
Sea Breeze - Sea breezes occur during the daytime in warm sunny weather when the air over a land is heated more rapidly than that over an adjacent water surface. As a result, the warmer air rises and relatively cool air from the sea flows onshore to replace it. At night, the air over the land cools faster than that over the nearby ocean and causes the air circulation to be in the opposite direction - a land breeze. Land breezes are usually weaker than sea breezes and have a less noticeable effect upon the temperature.
Severe Thunderstorm - A thunderstorm which produces tornadoes, large hail, or winds of 50 knots (80 kph) or more. Structural wind damage may imply the occurrence of a severe thunderstorm.
Shear - Variation in wind speed and/or direction) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.
Shortwave (or Shortwave Trough) - A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
Sounding - A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in weather forecasting, e.g., to determine instability, locate temperature inversions etc.
Squall Line - A solid or nearly solid line or band of active thunderstorms.
Steering Winds (or Steering Currents) - A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
Stratiform - Having extensive horizontal development, as opposed to the more vertical development characteristic of convection. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).
Stratocumulus - Low-level clouds, existing in a relatively flat layer but having individual elements. Elements often are arranged in rows, bands, or waves. Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.
Stratus - A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.
Subsidence - Sinking (downward) motion in the atmosphere, usually over a broad area ~ it often implies clearing skies.
Synoptic Scale (or Large Scale) - Size scale referring generally to weather systems with horizontal dimensions of several hundred miles or more. Most high and low pressure areas seen on weather maps are synoptic-scale systems. Compare with mesoscale.
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Temperature - In general, the degree of hotness or coldness measured against some definite scale by means of a thermometer.
Thermodynamics - In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.
Thunderstorm - (or thundershower). A local storm, produced by a cumulonimbus cloud, and accompanied by thunder and lightning.
Tornado - A tornado appears as a violent funnel-shaped wind vortex in the lower atmosphere with upward spiralling winds of high speeds - spawned by severe thunderstorms. The tornado usually appears from a bulge in the base of a cumulonimbus cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours. In area, it is one of the least extensive of all storms, but in violence, it is the world's most severe. More tornadoes occur in the United States than in any other country.
Trough- An elongated area of low atmospheric pressure associated with the jet stream. Low pressure and cooler weather are typical conditions of a trough.
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Urban Heat Island- The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas created when undeveloped land is developed covered in asphalt and concrete.
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Virga- Precipitation that falls from a cloud but evaporates before reaching the ground.
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Warm Advection - Transport of warm air into an area by winds.
Warm Front- A front that moves in such a way that warm air replaces cold air.
Warning - A product issued by Environment Canada forecast offices indicating that a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. Warnings are broadcast by the media, on the Weatheradio Canada System and by the Coast Guard. They are issued for snowstorms, blizzards, heavy blowing snow, heavy rains, frost, cold waves, freezing rain, severe thunderstorms and strong winds - according to thresholds established for local and regional public needs. In Vancouver a warning is issued for an expected snowfall of 5 centimeters or more while through most of Canada a warning would only be issued for 15 centimeters or more of snow.
Watch - An Environment Canada product indicating that a particular hazard is possible, i.e., that conditions are more favorable than usual for its occurrence. A watch is a recommendation for planning, preparation, and increased awareness (i.e., to be alert for changing weather, listen for further information, and think about what to do if the danger materializes).
Waterspout - In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud.
Wave - In meteorology, the intersection of warm and cold fronts.
Weather - State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day-to-day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.
Weather Balloon - large balloons filled with helium or hydrogen and carry radiosondes (weather instruments) aloft to measure temperature pressure and humidity as the balloon rises through the air. The whole contraption is attached to a small parachute so that when the balloon inevitably breaks, the radiosone doesn't hurtle back to earth dangerously quickly.
Wind - The horizontal movement of air relative to the earth's surface.
Wind Chill - The combined cooling effect of wind and temperature is called wind chill. The wind chill factor is a measure of this cooling effect. The larger the wind chill factor, the faster the rate of cooling. Note, however, that an object will not be cooled below the actual air temperature, it will just get there faster.
Windward - Upwind, or the direction from which the wind is blowing; the opposite of leeward.
